The present embodiments relate to a radio-frequency shielding unit for shielding a radio-frequency antenna unit of a magnetic resonance apparatus and to a magnetic resonance apparatus.
Magnetic Resonance Imaging (MRI) is a known technique for producing magnetic resonance images of a patient and is based on the physical phenomenon of magnetic resonance (MR). This technique uses a radio-frequency antenna unit to radiate excitation pulses into the patient during a magnetic resonance measurement in order to trigger magnetic resonance signals. For spatial encoding of the magnetic resonance signals, the magnetic resonance apparatus also includes a gradient coil unit that is used to generate magnetic field gradients. The magnetic field gradients are superimposed on a static main magnetic field B0 that is generated by a main magnet.
A transmit antenna that is fixedly integrated in the magnetic resonance apparatus is normally used as the radio-frequency antenna unit, and is often referred to as a whole-body radio-frequency coil or body coil for short. It is standard practice to bound the actual radio-frequency antenna unit by a radio-frequency shielding unit (e.g., a radio frequency shield, RF shield or shield). The documents U.S. Pat. No. 7,501,823 B2, U.S. Pat. No. 8,766,632 B2, and US 20130300418 A1 describe different exemplary embodiments of radio-frequency shielding units.
A radio-frequency shielding unit may include a ground plane and hence also a surface for image currents from the radio-frequency antenna unit. The radio-frequency shielding unit may be arranged between the radio-frequency antenna unit and the gradient coil unit, which is normally arranged further out with respect to a patient placement zone. The radio-frequency shielding unit is often mounted directly on an inside face of the gradient coil unit.
The radio-frequency shielding unit may conduct high frequency currents as effectively as possible in order to provide optimum efficiency of the radio-frequency antenna unit. Low-frequency eddy currents, which may be induced by fields from the gradient coil unit, are meant to be minimized as far as possible in order to prevent heating of the radio-frequency shielding unit. The fields from the eddy currents may extend even into the patient and cause there image interference and/or image artifacts in a magnetic resonance measurement.
Coupled-in noise (e.g., in the form of field spikes) that may be caused by switching pulses of gradient amplifiers, for example, may present another problem. Associated interference may likewise be attenuated by the radio-frequency shielding unit to avoid any impact on the imaging.
In order to satisfy all the requirements at once (e.g., high efficiency of the transmit antenna, sufficient suppression of eddy currents, and sufficient attenuation of interference), radio-frequency shielding units have long been in use that consist of printed circuit board material having copper layers applied on both sides (e.g., the front and back of the printed circuit board material is copper-coated). Both copper layers contribute to the shielding effect of the radio-frequency shielding unit.
In the past, radio-frequency shielding units of such a design have been well-suited to magnetic resonance apparatuses in which the main magnetic field B0 has a strength of typically 1.5 or 3 T. Such radio-frequency shielding units, however, exhibit significantly higher losses and significantly lower attenuating effects on external interference for magnetic resonance apparatuses in which the main magnetic field B0 is of lower strength (e.g., less than 1 T).